[Types-sig] Keyword arg declarations

[Guido van Rossum]

I just realized that Tim's decl syntax that's currently being bandied
around doesn't declare the names of arguments.  That's fine for a
language like C, but in Python, any argument with a name (*args
excluded) can be used as a keyword argument.

I think it will be useful for the decl syntax to allow leaving out or
supplying argument names -- that tells whether keyword arguments are
allowed for this particular function.  And that is part of a
function's signature.

(Note that not all builtins support keyword arguments; in fact most
don't.)

(Un)related: I think it makes sense to be able to restrict the types
of *varargs arguments.  E.g. eons ago (last week in the types-sig)
someone proposed an extension to isinstance() allowing one to write
isinstance(x, type1, type2, type3, ...).  Clearly the varargs are all
type objects here.

Not so sure about **kwargs, but these should probably be treated the
same way.

--Guido van Rossum (home page: http://www.python.org/~guido/)